#demo {
  position: relative;
  width: 100px;
  height: 100px;
  background: #fff;
  border: 1px #000 solid;
}

#demo::before {
  position: absolute;
  left: 100%;
  width: 0px;
  height: 0px;
  content: "";
  top: 20px;
  border-top: 10px solid transparent;
  border-right: 10px solid transparent;
  border-bottom: 10px solid transparent;
  border-left: 10px solid black;
}

#demo::after {
  position: absolute;
  left: 100%;
  width: 0px;
  height: 0px;
  content: "";
  top: 22px;
  border-top: 9px solid transparent;
  border-right: 9px solid transparent;
  border-bottom: 8px solid transparent;
  border-left: 8px solid white;
}